Don’t! If such a champagne is non-vintage from the peripheries of the region, the winemaker might have done a very good job and the wine might be very nice if drunk in the right situation. Let me emphasise again that these are subjective assessments: wines are living products which vary a good deal from bottle to bottle. Finally, the most important factor of all is your own taste. My assessments are not universal truths.
The Richard Juhlin 100 point scale
95-100
World Class champagne
90-94
Outstanding: a champagne of superior character and style
85-89
Very good: a champagne with special qualities
80-84
A really good champagne
70-79
High quality for non-vintage wines, ordinary for Vintage wines
60-69
Good non-vintage level
50-60
An ordinary non-vintage champagne
40-50
Not recommended
20-40
To be avoided
0-20
A disgrace for the district of Champagne